Electric field driven self-assembly of ionic microgels

We study, using fluorescent confocal laser scanning microscopy, the directed self-assembly of cross-linked ionic microgels under the influence of an applied alternating electric field at different effective packing fractions phi(eff) in real space. We present a detailed description of the contribution of the electric field to the soft interparticle potential, and its influence on the phase diagram as a function of phi(eff) and field strength E at a constant frequency of 100 kHz. In our previous work [Mohanty et al., Soft Matter, 2012, 8, 10819], we demonstrated the existence of field-induced structural transitions both at low and high phi(eff). In this work, we revisit the phase behavior at low and intermediate phi(eff) with a focus on both structure and dynamics. We demonstrate the existence of various field induced transitions such as an isotropic fluid to string phase to body centered tetragonal (BCT) crystal phase at low concentrations and a reversible field-induced crystal (face centered cubic, FCC) to crystal (BCT) transition at intermediate concentrations. We also investigate the kinetics of the crystal-crystal transition and demonstrate that this occurs through an intermediate melting process. These results are discussed in the light of previous studies of dipolar hard and charged colloids.